1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic process or the like for forming an image using a plurality of image bearing members (e.g., photosensitive members).
As the image forming apparatus of the electrophotographic process, there are, for example, an electrophotographic copying machine, an electrophotographic printer (e.g., an LED printer and a laser beam printer) and an electrophotographic facsimile machine.
2. Description of the Related Art
In recent years, with the spread of a color image forming apparatus according to the electrophotographic process, a demand for speed-up of color output has been increasing in addition to a demand for a recording quality of a color image. In order to meet these demands, there have been made several proposals with respect to an image forming process.
For example, a color image farming apparatus of a tandem type uses a plurality of photosensitive drums arranged side by side. The color image forming apparatus forms a toner image of each color of black, cyan, magenta and yellow on each photosensitive drum. Then, the toner images are sequentially transferred to a transfer material, which is conveyed by a transfer material conveyor belt, by respective transferring means. Thereafter, the toner images are fixed by a fixing device.
FIGS. 3 and 4 are sectional views of a conventional color image forming apparatus of the tandem type. As shown in FIGS. 3 and 4, transfer materials S are set in a sheet feeding cassette 101 inserted in a lower part of an apparatus main body. The transfer materials S are fed by a sheet feed roller 102 and sent to a registration roller pair 104.
There is provided a transfer material conveyor belt 105 for conveying the transfer materials S on a downstream side in a conveying direction of transfer materials of the registration roller pair 104. The transfer material conveyor belt 105 is stretched and suspended flatly in the conveying direction of transfer materials (vertical direction of FIG. 3) by a plurality of rotation rollers such as a drive roller 106, a driven roller 107 and a tension roller 108. An attracting roller 109 is arranged in a most upstream part in the conveying direction of transfer materials of the transfer material conveyor belt 105. The attracting roller 109 is brought into pressed contact with the driven roller 107 via the transfer material conveyor belt 105.
A predetermined bias voltage is applied to the attracting roller 109. Consequently, the transfer material S, which is conveyed onto the transfer material conveyor belt 105 at good timing by the registration roller pair 104, is electrostatically attracted by the transfer material conveyor belt 105 to be conveyed.
Four photosensitive drums of yellow, magenta, cyan and black 110y, 110m, 110c and 110k (hereinafter simply referred to as xe2x80x9cphotosensitive drums 110xe2x80x9d) are linearly arranged in order from the lower side to the upper side of FIG. 3 to be opposed to the transfer material conveyor belt 105.
Primary chargers 111y, 111m, 111c and 111k (hereinafter simply referred to as xe2x80x9cprimary chargers 111xe2x80x9d), developing devices 112y, 112m, 112c and 112k (hereinafter simply referred to as xe2x80x9cdeveloping devices 112xe2x80x9d) and cleaning means 113y, 113m, 113c and 113k (hereinafter simply referred to as xe2x80x9ccleaning means 113xe2x80x9d) are arranged around the respective photosensitive drums 110. In addition, transfer members 114y, 114m, 114c and 114k (hereinafter simply referred to as xe2x80x9ctransfer members 114xe2x80x9d) are arranged opposite to the photosensitive drums 110 across the transfer material conveyor belt 105.
Toner of each color is contained inside toner containers of the developing devices 112. Toner images of each color are formed on the photosensitive drums 110 by a well-known image forming process and are sequentially transferred to the transfer material S, which is conveyed by the transfer material conveyor belt 105, to form a color image.
The transfer material S having the toner image transferred thereon is self-stripped in the position of the drive roller 106 and guided to a fixing roller pair 115. The transfer material S is subjected to heating and pressurizing processing while it is nipped and conveyed by the fixing roller pair 115, and the toner image is permanently fixed. Thereafter, the transfer material S is conveyed by a discharge roller pair 116 and discharged onto a discharge tray 117 arranged in the upper part of the apparatus main body.
In the color image forming apparatus as described above, multicolor printing, that is, full color image formation is not always performed but a frequency of performing recording of single color of black, that is, monochrome image formation is also relatively high. Leaving developing rollers in contact with photosensitive drums that do not perform image formation even in such a case contributes to generation of fog or the like and is not preferable.
Therefore, in order to cope with such a method of use, there is proposed a color image forming apparatus of a tandem type that is constituted such that a contact or non-contact state of photosensitive drums and developing rollers can be appropriately switched in association with a case of full color image formation and a case of monochrome image formation.
For example, as shown in FIG. 3, image bearing members and developing rollers are in a contact state at the time of full color image formation. On the other hand, at the time of monochrome image formation, as shown in FIG. 4, switching means 118 (118y, 118m, 118c and 118k) for switching contact and non-contact of the image bearing members and the developing rollers push up a part of bottoms of developing devices to lift them upward in FIG. 4 to separate developing rollers 100 from photosensitive drums 110 other than that of black to make them non-contact. Thus, only a black image is recorded.
Constituting the color image forming apparatus in this way, a route of the transfer material S conveyed from the attracting roller 109 to the fixing roller pair 115 through a black image forming portion does not change in the case of full color image formation and the case of monochrome image formation, and a stable conveying performance is realized.
However, in the aforementioned conventional example, if a large number of monochrome images are formed, an abutment state of a cleaning blade tends to be unstable in process cartridges other than that for black. This is because, since the photosensitive drums and the developing rollers are not in contact with each other, toner hardly moves to the photosensitive drums and toner as lubricant decreases in rubbing parts of photosensitive drums and the cleaning blades. When the abutment state of the cleaning blades becomes unstable, it is likely that defective cleaning such as toner slipping or image defectiveness due to contamination of a charging member is caused. In the worst case, it is likely that damages such as tucking of cleaning blades are caused.
The present invention has been devised in view of the above and other drawbacks, and it is an object of the present invention to provide an image forming apparatus in which defective cleaning by cleaning means is prevented.
It is another object of the present invention to provide an image forming apparatus that is capable of performing image formation at a high speed using a plurality of image bearing members.
It is another object of the present invention to provide an image forming apparatus in which an abutment state of an image bearing member, on which an image is not formed, among a plurality of image bearing members and cleaning means is stabilized.
It is another object of the present invention to provide an image forming apparatus in which developer is supplied between an image bearing member on which an image is not formed and cleaning means.
It is yet another object of the present invention to provide an image forming apparatus that is capable of obtaining a high quality image steadily even if a large number of images are formed on some of a plurality of image bearing members.
Other objects and characteristics of the present invention will be more apparent by reading the following detailed descriptions with reference to the accompanying drawings.